The present invention relates to a display system for fish sonar in which two echo signals of different frequency are respectively displayed on the right and left portions of the screen of a cathode ray tube.
Generally, an ultrasonic wave of a high frequency is reflected by small fish in the surface of the sea. An ultrasonic wave of a low frequency is reflected by large fish in the deep portion of the sea. Therefore, the transducers of the fish sonar selectively use a high frequency or a low frequency for detecting small fish or large fish respectively.
A fish sonar having a cathode ray tube displays the echo signals received by one ultrasonic transducer on its screen. Therefore, when a fisherman wishes to observe both the high frequency and the low frequency sonar echo signals at the same time, the respective high and low frequency signals are simultaneously displayed by two separate fish sonars, or are alternately displayed by one fish sonar.
However, the cost of two fish sonars is high and the one fish sonar cannot display the high and low frequency signals at the same time.
In order to eliminate these deficiencies, a new fish sonar in which the high and low frequency echo signals are simultaneously displayed on vertically spaced portions of an electronically split screen of a cathode ray tube, has been recently developed.
In this fish sonar, however, it is difficult to compare the displays of the two signals with each other, because these displays are necessarily vertically compressed and vertically spaced apart.